Speech intelligibility can be reduced by background noises, which include loud, confusing, or distracting sounds. Hearing impaired persons often have particular difficulty discerning speech in noisy environments, but people without any hearing disorder can experience similar difficulties in environments with high noise levels.
Audio processing devices have used a variety of techniques for suppressing unwanted noise. One commonly used technique attenuates large amplitude audio signals for protecting against the reproduction of excessively loud noises. Another technique attenuates low frequencies of sound to help prevent a so-called "upward spread of masking" by low frequency noises, which reduces intelligibility of the higher frequency sounds.
For example, U.S. Pat. No. 4,061,875 to Freifeld et al. discloses an audio processor that incorporates an adjustable high pass filter to reduce low frequency noise components of an audio signal. The cut-off frequency of the high pass filter can be adjusted in steps from 0.25 to 1.5 kilohertz, and the rate of attenuation of the filter (i.e., the roll-off rate) can be adjusted at each cut-off frequency in steps of 6, 12, and 18 decibels per octave. Together, these two adjustments are used to discriminate against particular noises.
U.S. Pat. No. 4,792,977 to Anderson et al. discloses a hearing aid circuit having a series of state variable filters for controlling frequency response characteristics. The pass band of the filter series can be adjusted to attenuate predetermined low frequencies of noise. The state variable filters are implemented in an integrated circuit using capacitor loaded operational transconductance amplifiers and include separate external controls for varying respective outputs of a high pass filter, a low pass filter, and a variable slope filter. The high and low pass filters are both fourth order filters (e.g., four pole filters) made up of two cascaded second order filters. The external controls set frequency response characteristics by adjusting the cut-off frequencies of the high and low pass filters without substantially changing the respective shapes ("Q") of their frequency response curves.
Although a predetermined amount of attenuation of particular low frequencies of sound can help to prevent certain kinds of noise from masking higher frequencies that are more important to speech intelligibility, the amount of predetermined attenuation can be more or less than that required for optimally attenuating the noise. For example, if too little attenuation is provided, some masking remains. However, if too much attenuation is provided, the perceived sound quality is unnecessarily reduced. In the absence of masking noise, attenuation of the low frequencies also reduces intelligibility.
Audio processing devices have also been designed to attenuate low frequencies of sound as a function of noise energy. For example, U.S. Pat. No. 4,490,585 to Tanaka discloses a hearing aid in which a low frequency component of ambient sound is used to shift a cut-off frequency of a high pass filter. An increasing level of the low frequency sound is used to shift the cut-off frequency up to 1.5 kilohertz for attenuating loud noises within the low frequency spectrum. However, important speech information is also conveyed at frequencies much less than 1.5 kilohertz, and shifting the cut-off frequency of the high pass filter through this region reduces speech intelligibility as well as noise.
U.S. Pat. No. 3,927,279 to Nakamura et al. discloses a hearing aid in which both lower and higher frequency components of the acoustic spectrum are attenuated in response to the detection of sound energy at frequencies considered above and below frequencies required for speech. A band-rejection filter is used to isolate frequencies below 300 hertz and above 3000 hertz, and the energy content of the isolated bands is detected to form a control signal. Response characteristics of both a high pass filter and a low pass filter are varied by the control signal to attenuate high and low frequency noises.
However, the hearing aid of Nakamura et al., like the hearing aid of Tanaka, also attenuates frequencies that convey important speech information. For example, the hearing aid of Nakamura et al. attenuates to some degree the entire range of frequencies between 300 and 3000 hertz, which includes frequencies containing crucial information for identifying both consonants and vowels.